1. Field of the Invention
This invention relates to X-ray photographic devices.
2. Prior Art
The invention is directed to X-ray photographic devices having an X-ray tube equipped with a primary diaphragm for defining the X-ray beam and having a film holder or cassette which is moveable independently of the X-ray tube in any desired direction. The X-ray tube primary diaphragm limits the X-ray beam. At least one light beam source is spatially rigidly locatd with respect to the primary diaphragm and the X-ray tube. The light beam from that source is focused and is provided with means for limiting the light beam. A marking on the film cassette or on a component attached to the cassette, is designed such that by projecting the light beam properly with respect to the marking, an operator can determine whether the central beam of the X-ray will impinge upon the center of the film holder and whether the desired X-ray focus to film distance exists.
X-ray photographic devices of the above described type are primarily used in connection with bedridden patients. In such assemblies the X-ray apparatus is mobile, generally being wheel mounted. An independent film cassette is positioned underneath or adjacent the patient. Generally such devices are provided with a swing arm which carries the X-ray tube. Thus the X-ray tube can be readily positioned relative to the patient and, for adjustment of the beam, relative to the cassette. For the taking of X-rays, it is also known to use secondary beam rasters in order to reduce secondary radiation. Such rasters may, for example, consist of lead laminae positioned on edge and having a high X-ray beam absorption ability. The lead laminae may be surrounded by a medium having a low absorption. The raster assembly is then positioned between the patient and the film. This requires a specific film to focus distance for the specific raster being used. If the film to focus distance is not correctly adjusted, and if the center of the X-ray beam is not positioned properly with respect to the center of the raster, the picture providing X-radiation may be absorbed by the laminae along the width of the secondary radiation and a so-called raster effect formed. This raster effect is very interfering in practice and often requires, particularly with bedridden patients, the repetition of the taking of the X-ray photograph. This brings about an increased patient dosage of radiation, increased film consumption, and a waste of hospital personnel.
Therefore devices which provide for alignment of the film to focus distance and of the beam center of film center positioning have the ability to substantially reduce such disadvantages. A device having these capabilities as above described is known from U.S. Pat. No. 4,092,544, issued May 30, 1978, the teachings of which are herein incorporated by reference. In that construction the film holder or cassette is provided with a rigid laterally projecting portion. A marking is applied to that projecting portion. The marking includes a rectangular field. A light source rigidly positioned with respect to the X-ray tube has its light beam rectangularly limited. According to the teachings of that patent, the limited light beam is, prior to taking the X-ray, adjusted such that the marking on the cassette projection coincides with the rectangular light beam projection. When that occurs, the central beam of the X-ray will be aligned with the center of the film holder. Moreover the focus to film distance will also be correctly adjusted.
Although the above described device has the capability of eliminating many of the above-mentioned disadvantages, it has a noticeable disadvantage in that the rectangular marking on the cassette is relatively small such that it is difficult and time consuming to correctly adjust the light beam relative thereto. It would therefore be an improvement in the art to provide a simplified manner of properly locating the X-ray source with respect to the film in devices where the film is moveable independent of the X-ray source and is not fixed thereto.